On Windows (works on GNU/Linux) adding x500.bund.de as an S/MIME Certificate server makes searches in Kleopatra hang indefinitely or at least very long.
This was with latest GnuPG Stable branch (2.7 beta) and Kleopatra master.
I've also received a report (with screenshots) that dirmngr can crash in this configuration. (Crash was with 2.2.6 from Gpg4win-3.1.0)